Add some right-header pins at the end, have those plug into a tiny perpendicular board for the compass chip.

Have the socket for the motors' ribbon cable and the socket for the batteries dangle on external wires. This is slightly less solid of a design, but is more flexible and will make soldering MUCH easier.

If you arrange the shift register correctly under the RBBB, four out of five of the data pins between the two can just be bridged across without wires.

Soldering lessons:

If your brain is capacious enough, figure out maximal alignment before soldering anything, or you'll end up soldering obnoxious little wires for hours.

If you try to be too neat, however, you will waste a great deal of time soldering in impossibly tight corners, only likely to realize that you have planned your optimistically über-efficient circuit wrong and have to get out the solder-wick and go crazy...

(Notes on pin rearrangements breadboard vs. final board. This is just for my own reference.)